My 99 tj came from the factory with 4.10 gears and 205/75 tires (27 inch diameter). I now have 31s (about 30.5 diameter). Calculating the equation to get me back to stock ratio works out close to 4.56 gears yet everything I read says 4.56 is too low for 31s. What am I missing? Please help.

Install a tach and see what your spinning at your true target speed. Ie, if your looking to run at 55MPH (actual, your speedo will be slower due to the larger tires), see what the engine is turning. Then decide where the sweet spot in your torque curve is and try to target it.

I'd hold off on regearing. My 4cyl ran fine with 31's. You know thats not the end size tire, so wait if you can. I went to 33's and decided that was enough. The math for 33's on a 4 cyl says 4.88, so that's what I did. That was great for 33's. When I later went to 35's, it wasn't really a gear problem, but became a power problem. It was tough for the 4 cyl to spin the larger mass tires to accelerate, even though the tire size was only 6% larger. I then went to a V6 and kept the 4.88s. Life is great now (since the motor in 2004) on and off road.